The purpose of this open-label study is to confirm the beneficial effects of ucb L059 (levetiracetam) in children with partial onset epilepsy as well as to document the pharmacokinetic parameters and dose tolerability in the pediatric population. Clinical experience has shown uab L059 to be safe and well tolerated in adult patients with partial onset and generalized seizures after both acute and repeated administration, especially within the context of antiepileptic therapies. In previous clinical trials, the adverse experiences have been limited to dizziness, nervousness, sleepiness, weakness and headache; no detrimental effects on cognition have been observed with the administration of ucb L059. It is foreseen that the coadministration of ucb L059 will produce comparable benefits in efficacy and safety in this pediatric population. In this open-label trial of adjunctive ucb L059, safety and efficacy will be studied in patients 6-12 years (inclusive) with partial-onset seizures. The trial consists of four phases following a screening visit: pre-treatment Baseline, Titration, Evaluation and Withdrawal. The Baseline phase consists of a minimum of four weeks and a maximum of eight weeks on a single anticonvulsant agent. The Titration phase is six weeks in duration; concomitant anticonvulsants will remain constant throughout this phase. The Evaluation phase is eight weeks. Data collected during the Evaluation and Titration phases will be compared to the Baseline phase in determining the safety and efficacy of ucb L059. Daily diaries recording seizure type and frequency will be maintained throughout all phases of the study. Efficacy will be determined by comparing the mean number of seizures per week during the Baseline phase with the mean number of seizures per week during the Titration and Evaluation phases. The 20-item Hague Side Effects Scale will be completed by the parent/guardian at each visit. The 13-item Hague Seizure Severity Scale will be completed three times throughout the study. As an additional exploratory evaluation for efficacy, changes in the total score of the Hague Seizure Severity Scale and the Side Effects Scale will be compared with the Baseline period. Safety assessment will consist of evaluation of clinical laboratory tests. EKGs, vital signs, physical and neurological examinations and documentation of adverse experiences.